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P0300 is random misfires. Meaning more than one cylinder was misfiring. Year make model and engine size helps. On 4 cylinders 1 and 4 fire at the same time, this would be an ignition module. On a lot of models.

HARD TO SAY WITH OUT BEING THERE BUT THIS MIGHT HELP .
Crankshaft sensor Was the trouble with my 99 Chevy truck after replacing many components like coil cap rotor wires and so on . http://www.obd-codes.com/p0102
Code P0300 Causes and Fixes
Code p0300 is a code that is generated when your engine has a misfire that is not related to only one cylinder. Sometimes this code will be set in addition to other codes such as p0301 which means that there is a misfire on cylinder 1 (cylinder 1 is the number one cylinder when looking at the firing order. It is generally the forward most cylinder on the engine.). If these engine codes are present as well the you should diagnose those first as they are probably causing code p0300.
This code will set your check engine light and needs to be fixed as soon as possible. If it is not then you will probably need to replace your catalytic converter as well as the problem that is causing your misfire.
There are many causes for this obd code. Since it is a "random misfire" or "multiple misfire" this means that the misfire is on different cylinders and not just one all the time.
Common Causes

A P0300 code indicates random, multiple misfires were detected. You eliminated the possibility of ignition system faults by replacing the ignition module, the coils, and the spark plugs (unless the replacement part(s) were no good). However you did not mention the ignition wires which are another probable cause. In addition, there could be vacuum leaks due to damaged hoses or improper routing. Consider also fuel contamination or a problem with the fuel system.

Without trouble codes you are fishing in the dark. If the engine has a cylinder that is dead at idle from a fuel injector or ignition problem, it should have a code for a miss on that cylinder. A mechanical problem like a burned valve could cause the problem without setting a code. Have you checked compression on all 4 cylinders ? ?

It may just be time for spark plugs for all we know. Not knowing the history thats where I'd begin, Then injectors and maybe a coil if were guessing. Now if you dont want to guess get it diagnosed properly.